Re: Best cap size for a sync gap

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Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz> 

Hi Gary,
           Nice expt:

On 28 Apr 2004, at 21:57, Tesla list wrote:

 > Original poster: "Lau, Gary" <gary.lau-at-hp-dot-com>
 >
 > I finally "got around to it" to an experiment that's been nagging me
 > for a very long time.  I wanted to determine the optimum cap size for
 > a coil, in terms of maximizing the power pulled from the NST.  Tank
 > frequency issues come later.
 >
 > With a newly constructed sync RSG and a dummy load consisting of three
 > 500W halogen lamps in series in place of the primary inductor, I
 > scoped the gap voltage.  As is typical with sync gaps, I adjusted the
 > phase such that the bang occurs somewhat after the peak charging
 > voltage.  The later I phased it, the brighter the lamps and the higher
 > the bang voltage. If the timing is too late, the gap stops firing
 > altogether, so I brought it just to the brink of this point.
 >
 > To measure the voltage, I use Terry's fiber optic probe, which sadly,
 > I have yet to accurately calibrate.  But for the purposes of finding
 > the best cap size, even a qualitative measurement is adequate.
 >
 > I use an unmodified 15/60 NST, cranked up to 144VAC.  I had available
 > two .02uF caps, and one .01uF cap.  With these I measured the bang
 > voltages using .02, .03, .04, and .05uF.  The peak-to-peak bang
 > voltages measured were 688, 618, 552, and 482 mV respectively, as
 > directly indicated on the scope.  FWIW, if I scope just the unloaded
 > NST secondary -at-120VAC input, I get 598mV p-p.
 >
 > If I calculate the relative bang size with a simple scale-less formula
 > of C*V*V (mV*uF*uF), I get .02   9,467 .03  11,478 .04  12,188 .05
 > 11,616
 >
 >  >From this I conclude that using a .04uF cap with my 15/60 NST will
 >  result
 > in the highest power throughput.
 >
 > Has anyone else performed such an experiment?  Just trying to
 > understand why my result is so at odds with the widely suggested value
 > of .028uF for the same power supply.  Hmmm, wonder what I'd have
 > gotten if I had tested at 120VAC?
 >
 > Regards, Gary Lau
 > MA, USA

There is a possible fly in the ointment and by proxy, load dependency
which is not that obvious. Consider that the cap charging current is
running through the lamp filaments and that the resistance of these
is temperature-dependent. Your thoughts?

Malcolm